Semiconductor memory device and data write method thereof

ABSTRACT

A semiconductor memory device includes a control circuit. The control circuit executes control to perform a verify operation with respect to only a lowest threshold voltage level of a memory cell at a time of a data write operation, and to skip the verify operation with respect to the other threshold voltage levels. The control circuit determines whether a verify pass bit number of the lowest threshold voltage level, which is counted by a bit scan circuit, is a prescribed bit number or more, and the control circuit further executes control, if the verify pass bit number is the prescribed bit number or more, to perform the verify operation with respect to only the lowest threshold voltage level and a threshold voltage level that is higher than the lowest threshold voltage level, and to skip the verify operation with respect to the other threshold voltage levels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 12/535,040 filed Aug. 4, 2009,now U.S. Pat. No. 7,948,804 and claims the benefit of priority under 35U.S.C. §119 from Japanese Patent Application No. 2008-230295 filed Sep.8, 2008, the entire contents of each of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device and adata write method thereof, and is applied, for example, to a NAND flashmemory.

2. Description of the Related Art

In a data write operation of a NAND flash memory, for example, a programloop is repeated with an arbitrary step width, and the program loop isstopped when data write in all bits is finished.

With the development of multilevel implementation of NAND flashmemories, it becomes necessary to more finely control the intervals ofdata write levels of memory cells. It is thus necessary to make a writestep voltage (dVPGM) lower. However, the decrease in write step voltage(dVPGM) leads to an increase in the number of times of data write, thatis, an increase in program time (tPROG).

In the market in recent years, there has been a stronger demand forhigher-speed, larger-capacity memories. In this trend, one problem ishow to shorten a write time, while ensuring reliability.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided asemiconductor memory device comprising: a memory cell array including aplurality of memory cells which are arranged in a matrix atintersections between a plurality of word lines and a plurality of bitlines; a word line driving circuit which applies a voltage to the wordlines; a bit line control circuit including a buffer which retains averify result of the memory cell array, and a bit scan circuit whichcounts a verify pass bit number on the basis of the verify result thatis retained in the buffer; and a control circuit which controls the wordline driving circuit and the bit line control circuit, the controlcircuit executing control to perform a verify operation with respect toonly a lowest threshold voltage level of the memory cell at a time of adata write operation, and to skip the verify operation with respect tothe other threshold voltage levels.

According to another aspect of the present invention, there is provideda semiconductor memory device comprising: a memory cell array includinga plurality of memory cells which are arranged in a matrix atintersections between a plurality of word lines and a plurality of bitlines, and are capable of storing multi-bit data; a word line drivingcircuit which applies a voltage to the word lines; a bit line controlcircuit including a buffer which retains a verify result of the memorycell array, and a bit scan circuit which counts a verify pass bit numberon the basis of the verify result that is retained in the buffer; and acontrol circuit which controls the word line driving circuit and the bitline control circuit, the control circuit executing control to perform averify operation with respect to only a first threshold voltage levelwhich is higher than a lowest threshold voltage level of the memory cellat a time of a data write operation, and to skip the verify operationwith respect to the other threshold voltage levels.

According to still another aspect of the present invention, there isprovided a data write method of a semiconductor memory device,comprising: executing a verify operation with respect to only a lowestthreshold voltage level of a memory cell, and skipping the verifyoperation with respect to the other threshold voltage levels; counting averify pass bit number of the lowest threshold voltage level;determining whether the counted verify pass bit number of the lowestthreshold voltage level is a prescribed bit number or more; andexecuting, if the verify pass bit number is the prescribed bit number ormore, the verify operation with respect to only the lowest thresholdvoltage level and a threshold voltage level that is higher than thelowest threshold voltage level, and skipping the verify operation withrespect to the other threshold voltage levels.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing an example of the entire structure ofa semiconductor memory device according to a first embodiment of thepresent invention;

FIG. 2 is a block diagram showing a structure example of a bit linecontrol circuit in FIG. 1;

FIG. 3 is an equivalent circuit diagram showing a structure example of ablock in FIG. 1;

FIG. 4 is a plan view showing a plan-view structure example of a memorycell array in FIG. 1;

FIG. 5 is a cross-sectional view showing a structure example of a crosssection taken along line V-V in FIG. 4;

FIG. 6 is a view showing a threshold value distribution of thesemiconductor memory device according to the first embodiment;

FIG. 7A is a flow chart illustrating data write of the semiconductormemory device according to the first embodiment;

FIG. 7B is a flow chart illustrating data write of the semiconductormemory device according to the modification 1;

FIG. 8 is a view showing the relationship between write loops and thenumber of times of verify in a case of verify skip;

FIG. 9 is a view showing the relationship between the write loops andthe number of times of verify in a case of normal skip;

FIG. 10 is a timing chart showing a voltage waveform of a select wordline in the case of normal skip;

FIG. 11 is a view showing the relationship between the number of programloops and a threshold voltage;

FIG. 12 is a view showing the relationship between a write voltage and athreshold voltage;

FIG. 13 is a view for explaining a verify state in the first embodiment;

FIG. 14 is a view showing the relationship between the write loop andthe number of times of verify in a case of “Initial”;

FIG. 15 is a view showing the relationship between the write loop andthe number of times of verify in a case of “After Write/Erase”;

FIG. 16 is a view showing the relationship between the write loop andthe number of times of verify in a final state of verify skip;

FIG. 17A is a flow chart illustrating data write of the semiconductormemory device according to the first embodiment;

FIG. 17B is a flow chart illustrating data write of the semiconductormemory device according to the modification 2;

FIG. 18 is a view showing the relationship between the write loop andthe number of times of verify in a final state of improved verify skip;

FIG. 19 is a view showing the relationship between the write loop andthe number of times of verify in the “Initial” state of improved verifyskip;

FIG. 20 is a view showing the relationship between the write loop andthe number of times of verify in the case of “After Write/Erase” ofimproved verify skip;

FIG. 21 is a timing chart showing a voltage waveform of a select wordline in the improved verify skip;

FIG. 22 is a view showing the relationship between the write loop andthe number of times of verify in a semiconductor memory device accordingto a second embodiment of the invention;

FIG. 23 is a view showing a threshold value distribution in one writeloop of the semiconductor memory device according to the secondembodiment;

FIG. 24 is a view showing a threshold value distribution in one writeloop of the semiconductor memory device according to the secondembodiment;

FIG. 25 is a view showing a threshold value distribution in one writeloop of the semiconductor memory device according to the secondembodiment; and

FIG. 26 is a view showing a threshold value distribution in one writeloop of the semiconductor memory device according to the secondembodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings. In the description below, commonparts are denoted by like reference numerals throughout the drawings. Inaddition, in the description of the embodiments, NAND flash memories aretaken as examples of semiconductor memory devices.

Data write of a select word line of a NAND flash memory is therepetition of application of write pulses (Program) and verify pulses(Verify) (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2002-140899).

Further, in a multilevel NAND flash memory (MLC: multilevel cell), thetime of application of verify pulses (Verify) occupies most of the writetime. Thus, from the standpoint of reduction in data write time, it isimportant to reduce the number of times of application of verify pulses(Verify) as much as possible, while maintaining reliability.

However, in the above-described data write operation with the generalstructure, the application of verify pulses is executed at all thresholdvoltage levels, and this is disadvantageous in reduction of program time(tPROG).

Thus, semiconductor memory devices and the data write methods thereof,which are advantageous in reducing the program time (tPROG), areconcretely described with reference to the following embodiments.

First Embodiment 1. Structure Example

To begin with, referring to FIG. 1 to FIG. 6, a description is given ofa structure example of the NAND flash memory according to the firstembodiment of the present invention.

1-1. Example of Entire Structure

FIG. 1 shows an example of the entire structure of the NAND flash memoryaccording to the first embodiment.

As shown in FIG. 1, the NAND flash memory according to this embodimentcomprises a memory cell array 11, a bit line control circuit 12, acolumn decoder 13, a data input/output buffer 14, a data input/outputterminal 15, a word line driving circuit 16, a control circuit 17, and acontrol signal input terminal 18.

The memory cell array 11 is composed of a plurality of blocks (BLOCK1 toBLOCKn). The word line driving circuit 16 for controlling word lines andthe bit line control circuit 12 for controlling bit lines are connectedto the memory cell array 11.

The bit line control circuit 12 reads data of memory cell transistors inthe memory cell array 11 via the bit lines, and detects the states ofthe memory cell transistors in the memory cell array 11 via the bitlines. In addition, the bit line control circuit 12 executes data writein the memory cell transistors by applying write control voltages to thememory cell transistors in the memory cell array 11 via the bit lines.The column decoder 13 and the data input/output buffer 14 are connectedto the bit line control circuit 12.

Data memory circuits (not shown) are provided in the bit line controlcircuit 12, and the data memory circuits are selected by the columndecoder 13. The data of the memory cell transistors, which are read intothe data memory circuits, are output to the outside from the datainput/output terminal 15 via the data input/output buffer 14. The datainput/output terminal 15 is connected to, for example, an external hostdevice (or memory controller).

The host device is, for instance, a microcomputer, which receives datathat is output from the data input/output terminal 15. Further, the hostdevice outputs various commands CMD which control the operation of theNAND flash memory, addresses ADD and data DT. Write data, which is inputto the data input/output terminal 15 from the host device, is deliveredvia the data input/output buffer 14 to the data memory circuits (notshown) which are selected by the column decoder 13. On the other hand,the commands and addresses are delivered to the control circuit 17.

The word line driving circuit 16 selects the word lines in the memorycell array 11, and applies voltages necessary for data read, write orerase to the selected word lines.

The control circuit 17 delivers necessary control signals and controlvoltages to the respective circuits that are connected. The controlcircuit 17 is connected to the memory cell array 11, bit line controlcircuit 12, column decoder 13, data input/output buffer 14 and word linedriving circuit 16. These connected structural circuits are controlledby the control circuit 17. The control circuit 17 is connected to thecontrol signal input terminal 18, and is controlled by control signals,such as an ALE (address latch enable) signal, which are input from theexternal host device (or memory controller) via the control signal inputterminal 18. In addition, the control circuit 17 includes an internalvoltage generating circuit, and supplies control voltages which arenecessary for the operations of the connected structural circuits.

The word line driving circuit 16, bit line control circuit 12, columndecoder 13 and control circuit 17 constitute a write circuit, a readcircuit and an erase circuit.

1-2. Structure Example of the Bit Line Control Circuit

Next, referring to FIG. 2, a description is given of a structure exampleof the bit line control circuit 12 according to the first embodiment.

As shown in FIG. 2, the bit line control circuit 12 according to thisembodiment includes a page buffer 21, a bit scan circuit 22 and a cachememory 23.

The page buffer 21 retains, for example, a verify result at a time of adata write operation, which will be described later.

The bit scan circuit 22 counts a verify pass bit number on the basis ofthe verify result that is retained in the page buffer 21. To be morespecific, at the time of the data write operation, the bit scan circuit22 scans the data stored in the page buffer 21, which corresponds topage data.

At the time the verify operation, with respect to the data in the pagebuffer 21, which corresponds to the memory cell transistors in whichdata write is completed, inversion from “0” data to “1” data is executedin order to prevent further write. The data write operation is executeduntil all data that are retained in the page buffer 21 become “1” data.

The bit scan circuit 22 can scan not only the above-described bits(verify pass bits) for which data write is completed, but also bits(fail bits) for which data write is not completed. To be more specific,fail bits can be detected by executing detection in a manner reverse tothe general detection in the bit scan circuit 22, for example, byaltering the bit scan method in a manner to execute, e.g. binary scan.

The cache memory 23 is connected to the column decoder 13, andtemporarily stores write/read data which is input from the columndecoder 13.

The page buffer 21 and the cache memory 23 constitute a sense amplifier25.

1-3. Structure Example of Block (BLOCK)

Next, a structure example of the blocks (BLOCK), which constitute thememory cell array according to the first embodiment, is described withreference to FIG. 3. In this description, one block (BLOCK1) in FIG. 1is taken as an example. The memory cell transistors in the block BLOCK1are erased batchwise. Thus, the block is an erase unit.

The block BLOCK1 is composed of a plurality of memory cell units MUwhich are arranged in the word line direction (WL direction). The memorycell unit MU comprises a NAND string (memory cell string), which iscomposed of eight memory cell transistors MT0 to MT7 that are arrangedin the bit line direction (BL direction) perpendicular to the WLdirection and have series-connected current paths, a source-side selecttransistor S1 which is connected to one end of the current path of theNAND string, and a drain-side select transistor S2 which is connected tothe other end of the current path of the NAND string.

In this example, the memory cell unit MU is composed of eight memorycells MT0 to MT7. However, the memory cell unit MU may be composed oftwo or more memory cells, for instance, 16 memory cells or 32 memorycells, and the number of memory cells is not limited to eight.

The other end of the current path of the source-side select transistorS1 is connected to a source line SL. The other end of the current pathof the drain-side select transistor S2 is connected to a bit line BLm−1,which is provided on the upper side of the memory cell unit MU inassociation with each memory cell unit MU, and extends in the BLdirection.

Word lines WL0 to WL7 extend in the WL direction, and are commonlyconnected to control gate electrodes of a plurality of memory celltransistors which are arranged in the WL direction. A select gate lineSGS extends in the WL direction, and is commonly connected to aplurality of select transistors S1 which are arranged in the WLdirection. Similarly, a select gate line SGD extends in the WLdirection, and is commonly connected to a plurality of selecttransistors S2 which are arranged in the WL direction.

A page (PAGE) is present in each of the word lines WL0 to WL7. Forexample, as indicated by a broken-line box in FIG. 3, a page (PAGE) ispresent in the word line WL7. Since a read operation and a writeoperation are executed in units of the page (PAGE), the page (PAGE) is aread unit and a write unit.

1-4-1. Plan-View Structure Example of the Memory Cell Array

Next, referring to FIG. 4, a plan-view structure example of the memorycell array 11 is described. As shown in FIG. 4, each of the pluralblocks (BLOCK1, BLOCK2, . . . ), which constitute the memory cell array11, includes a plurality of memory cell transistors which are arrangedin a matrix at intersections between the plural word lines WL0 to WL7and device regions which are divided by device isolation regions andextend in the BL direction, and select transistors S1 and S2 which arearranged at intersections between the select gate lines SGS and SGD andthe device regions. The device isolation region is formed by burying,e.g. a silicon oxide film in a surface portion of the semiconductorsubstrate.

Source line contacts SC and bit line contacts BC are formed on deviceregions on both sides of the memory cell unit which is composed ofplural memory cell transistors and select transistors S1 and S2. Thesource line contacts SC and bit line contacts BC are shared by blocksneighboring in the BL direction. For example, the source line contactsSC are shared by the block BLOCK2 and block BLOCK3 which neighbor in theBL direction, and the bit line contacts BC are shared by the blockBLOCK1 and block BLOCK2 which neighbor in the BL direction.

The source line SL is provided on the source line contacts SC, andsub-bit lines SBL are provided on the bit line contacts BC. Inter-wiringcontacts LC are provided on the sub-bit lines SBL, and the bit lines BL0to BLm−1 are provided on the inter-wiring contacts LC.

1-4-2. Cross-Sectional Structure Example of the Memory Cell Array

Next, referring to FIG. 5, a description is given of a cross-sectionalstructure example of the memory cell array according to the firstembodiment. In this description, a cross-sectional structure example ina direction of the bit line BL3 along line V-V in FIG. 4 is taken as anexample.

As shown in FIG. 5, a memory cell unit MU is disposed on a device regionof a semiconductor substrate (Si-sub) 31. The memory cell unit MUincludes a plurality of memory cell transistors MT0 to MT7, which haveneighboring current paths and are connected in series, and selecttransistors S1 and S2 which select the memory cell transistors MT0 toMT7.

Each of the memory cell transistors MT0 to MT7 has a multi-layerstructure comprising a tunnel insulation film Tox, a floating electrode,FG0 to FG7, an inter-gate insulation film IPD, a control electrode, CG0to CG7 (WL0 to WL7), and a gate mask layer GM, which are successivelyformed on the semiconductor substrate. Each of the memory celltransistors MT0 to MT7 has a source S and a drain D, which are provided,spaced apart, in the semiconductor substrate 31 in a manner to sandwichthe multi-layer structure.

The select transistor S1 is disposed on the source side of the NANDstring which is configured such that the current paths of the memorycell transistors MT0 to MT7 are connected in series. The selecttransistor S1 includes, in the named order on the semiconductorsubstrate 31, a gate insulation film Gox, a gate electrode SG1, a gatecap layer GM, an inter-gate insulation film IPD, which is split at itscentral part in the gate electrode SG1, with the upper and lower layersof the inter-gate insulation film IPD being electrically connected, anda source S and a drain D which are provided, spaced apart, in thesemiconductor substrate 31 in a manner to sandwich the gate electrodeSG1.

The select transistor S2 is disposed on the drain side of the NANDstring. The select transistor S2 includes, in the named order on thesemiconductor substrate 31, a gate insulation film Gox, a gate electrodeSG2, a gate cap layer GM, an inter-gate insulation film IPD, which issplit at its central part in the gate electrode SG2, with the upper andlower layers of the inter-gate insulation film IPD being electricallyconnected, and a source S and a drain D which are provided, spacedapart, in the semiconductor substrate 31 in a manner to sandwich thegate electrode SG2.

A source line contact SC is provided on the source S/drain D of theselect transistor S1. A bit line contact BC is provided on the sourceS/drain D of the select transistor S2.

The source line contact SC, bit line contact BC, source line SL, sub-bitline SBL, inter-wiring contact LC and bit line BL3 are provided in aninterlayer insulation film 32.

Although not shown, a P-well or an N-well may be provided in thesemiconductor substrate 31. The P-well or N-well is formed by dopingP-type impurities or N-type impurities in the semiconductor substrate31.

The control gate electrodes CG0 to CG7 (or word lines WL0 to WL7) andthe select gates SGS and SGD are electrically connected to the controlcircuit 17 via a row decoder which is disposed in the word line drivingcircuit 16. A transfer gate is provided in the row decoder. An addressselect signal line is provided on the gate electrode of the transfergate. A voltage, which is generated in the control circuit 17, isapplied to any one of the memory cell transistors MT0 to MT7, which hasa selected address.

The description in this section 1-4-2 is given of the structure examplein which the gate insulation film Gox and tunnel insulation film Tox areprovided in association with each of the select transistor S1, S2 andthe memory transistor, MT0 to MT7. The invention, however, is notlimited to this structure example. For instance, such a structure may beadopted that the gate insulation film Gox and tunnel insulation film Toxmay be disposed over the semiconductor substrate 31 and may be providedcommonly to the select transistors S1 and S2 and the memory transistorsMT0 to MT7.

1-5. Example of Threshold Value Distribution

Next, referring to FIG. 6, a description is given of the distribution ofthreshold voltages of the NAND flash memory according to the firstembodiment.

As shown in FIG. 6, the threshold voltage (Cell Vth) of the NAND flashmemory according to this embodiment has a plurality of threshold voltagelevels Lev00 to Lev15. Thus, the NAND flash memory according to thepresent embodiment is a multilevel NAND flash memory (MLC: multilevelcell) which can store a plurality of bits in one memory cell transistorMT. A 16-value multilevel NAND flash memory is described below by way ofexample.

2. Data Write Operation

2-1A. Data Write Operation (Verify Skip)

Next, referring to FIG. 7A to FIG. 11, a description is given of a datawrite operation (Verify Skip) of the nonvolatile semiconductor memorydevice according to the first embodiment. The description below is givenwith reference to a flow chart of FIG. 7A. The data write operation tobe described below is a verify operation (hereinafter referred to as“Verify Skip”) in which a verify operation relating to a predeterminedthreshold voltage level is omitted, as compared to a verify operation(hereinafter “Normal Verify”) in which a verify operation is executedfor all the threshold voltage levels.

(Step S1-1 (Program))

To start with, as shown in FIG. 7A, the control circuit 17 executescontrol to apply a write pulse, on the basis of write data which isinput from the external host device via the data input/output terminal15.

(Step S1-2 (PL=N_(L)?))

Then, the control circuit 17 determines whether the number of times ofapplication of write pulses (the number of write loops) PL is aprescribed number N_(L) or not. The prescribed number N_(L) is a valuewhich is determined on the basis of a measurement result indicating thata bit at which data write is finished does not appear until the numberof application of write pulses reaches the prescribed number N_(L).Specifically, as long as the number of write loops is small, the verifyoperation is skipped with respect to all threshold levels.

(Step S1-3 (VPGM++, PL++))

In step S1-2, when the number of application of write pulses is not theprescribed number N_(L) (No), the control circuit 17 steps up the writevoltage VPGM, and increments the number of write loops by one, followingwhich the above-described step S1-1 and step S102 are executed onceagain. To be more specific, to step up the write voltage VPGM means thewrite voltage VPGM=VPGM_(init)+dVPGMX N_(Loop) (VPGM_(init): an initialvalue, dVPGM: a step-up width, N_(Loop): the number of loops).

(Step S1-4 (Program))

In step S1-2, if the number of application of write pulses, PL, is theprescribed number N_(L) (Yes), the control circuit 17 applies writepulses on the basis of the write data that is input from the externalhost device via the data input/output terminal 15.

(Step S1-5 (using Verify Skip))

Subsequently, the control circuit 17 controls the verify operation, anddetermines whether the verify operation can be completed or not. At thistime, the verify operation (Verify Skip) is executed, in which apredetermined verify operation is skipped, compared to the veryoperation (Normal Verify) in which a verify operation is executed forall threshold voltages. This verify operation (Verify Skip) will bedescribed later.

If the verify operation can be completed (Pass), the data writeoperation is finished.

(Step S1-6 (VPGM++, PL++))

If the verify operation cannot be completed (Fail), the write voltageVPGM is stepped up in the same manner as described above, and the numberof write loops is incremented by one, following which theabove-described steps S1-4 and S1-5 are executed once again.

2-1B. Data Write Operation (Verify Skip: Modification 1)

Next, referring to FIG. 7B, a description is given of a data writeoperation (Verify Skip) of the nonvolatile semiconductor memory deviceaccording to the modification 1. The description below is given withreference to a flow chart of FIG. 7B.

(Step S1-1 (PL=N_(L)?))

To start with, as shown in FIG. 7B, the control circuit 17 determineswhether the number of times of application of write pulses (the numberof write loops) PL is a prescribed number N_(L) or not. The prescribednumber N_(L) is a value which is determined on the basis of ameasurement result indicating that a bit at which data write is finisheddoes not appear until the number of application of write pulses reachesthe prescribed number N_(L). Specifically, as long as the number ofwrite loops is small, the verify operation is skipped with respect toall threshold levels.

(Step S1-2 (Program))

In step S1-1, when the number of application of write pulses PL, is notthe prescribed number N_(L) (No), the control circuit 17 executescontrol to apply a write pulse, on the basis of write data which isinput from the external host device via the data input/output terminal15.

(Step S1-3 (VPGM++, PL++))

Then, the control circuit 17 steps up the write voltage VPGM, andincrements the number of write loops by one, following which theabove-described step S1-1 and step S102 are executed once again. To bemore specific, to step up the write voltage VPGM means the write voltageVPGM=VPGM_(init)+dVPGM×N_(Loop) (VPGM_(init): an initial value, dVPGM: astep-up width, N_(Loop): the number of loops).

(Step S1-4 (Using Verify Skip))

In step S1-1, if the number of application of write pulses PL, is theprescribed number N_(L) (Yes), the control circuit 17 controls theverify operation, and determines whether the verify operation can becompleted or not. At this time, the verify operation (Verify Skip) isexecuted, in which a predetermined verify operation is skipped, comparedto the very operation (Normal Verify) in which a verify operation isexecuted for all threshold voltages. This verify operation (Verify Skip)will be described later.

If the verify operation can be completed (Pass), the data writeoperation is finished (End).

(Step S1-5 (Program))

If the verify operation cannot be completed (Fail), the control circuit17 applies write pulses on the basis of the write data that is inputfrom the external host device via the data input/output terminal 15.

(Step S1-6 (VPGM++, PL++))

Then, write voltage VPGM is stepped up in the same manner as describedabove, and the number of write loops is incremented by one, followingwhich the above-described step S1-4 is executed once again.

2-2. Verify Operation (Verify Skip)

Next, referring to FIG. 8 to FIG. 11, a description is given of theverify skip (Verify Skip) which is executed in the above-described FIG.7A and FIG. 7B. In this description, comparison is made with the normalverify (Normal Verify) in which a verify operation is executed for allthreshold voltage levels.

The Number of Times of the Verify Operation in the Case of the VerifySkip

To begin with, referring to FIG. 8, a description is given of the numberof verify operations in the case of the verify skip relating to theabove-described step S1-5. The relationship between the write loops(Program Loop) and the threshold voltage level (Lev00 to lev15) in thecase of the verify skip is as shown in FIG. 8.

As shown in FIG. 8, compared to the normal verify (Normal Verify) inwhich a verify operation is executed for all threshold voltage levels,Verify VC1 and Verify VC2, which are indicated by broken lines in FIG.8, can be omitted (“Skip”). Thus, at the time of the data writeoperation, the program time can advantageously be shortened.

For example, in the case of the present example, under the suppositionthat the write at the threshold voltage level Lev00 will be finishedfrom the fourth program loop (Prog Loop 4), a verify operation isexecuted for only the threshold voltage level Lev00 at the fourthprogram loop (the prescribed number N_(L)=3 is set). On the other hand,at the time of the fourth program loop (Prog Loop 4), the verifyoperation for the other threshold voltage levels Lev01 to Lev15 is notexecuted and is skipped.

For example, in the same manner as described above, at the time of thefifth program loop (Prog Loop 5), the verify operation is executed foronly the threshold voltage levels Lev00 and Lev01. On the other hand, atthe time of the fifth program loop (Prog Loop 5), the verify operationfor the other threshold voltage levels Lev02 to Lev15 is not executedand is skipped.

For example, at the time of the eighth program loop (Prog Loop 8), whichis indicated by an arrow in FIG. 8, the verify operation is executed forthe threshold voltage levels Lev00 to Lev04. On the other hand, theverify operation for the other threshold voltage levels Lev05 to Lev15is not executed and can be skipped.

Subsequently, in the same manner as described above, as the number ofprogram loops increases, the threshold level for the verify operation isincreased.

In the 23rd program loop (Prog Loop 23) and the following program loops,the verify operation is not executed for lower threshold voltage levels.For example, at the time of the 23rd program loop, the verify operationfor the threshold voltage level Lev00 is not executed on the assumptionthat the write at the threshold voltage level Lev00 is already finished,and the verify operation for the threshold voltage levels Lev01 to Lev15is executed.

Similarly, at the time of the 24th program loop (Prog Loop 24), theverify operation for the threshold voltage levels Lev01 and Lev02 is notexecuted on the assumption that the write at the threshold voltagelevels Lev01 and Lev02 is already finished, and the verify operation forthe threshold voltage levels Lev03 to Lev15 is executed.

Subsequently, in the same manner as described above, as the number ofprogram loops increases, the threshold level for the verify operation isdecreased.

The above-described verify operation can be realized, for example, bypresetting the “write loop number at which the verify operation isstarted” and the “write loop number at which the very operation isfinished” with respect to each of the threshold levels.

In this manner, since the write voltage is stepped up with fixed steps(dVPGM), the threshold level, which is the object of the verifyoperation, can be increased stepwise in the initial stage of data write(Prog Loop 4 to Prog Loop 19 in FIG. 8), the threshold level, which isthe object of the verify operation, is decreased stepwise in the laststage of data write (Prog Loop 22 to Prog Loop 37 in FIG. 8). This pointwill be described later.

As has been described above, as a result, the number of times of verifyin the case of the verify skip in this embodiment can be set at, e.g.304 in total.

The Number of Times of the Verify Operation in the Case of the NormalVerify

Next, referring to FIG. 9, a description is given of the number of timesof verify in the case of executing the normal verify, for the purpose ofcomparison with the number of times of verify in the case of executingthe verify skip. The relationship between the write loop (Program Loop)and the threshold voltage level (Lev00 to lev15) in this case is asshown in FIG. 9.

As shown in FIG. 9, in the normal verify (Normal Verify), the verifyoperation is executed for all the threshold voltage levels Lev00 toLev15.

For example, in the fourth program loop (Prog Loop 4), the verifyoperation is executed for all threshold voltage levels Lev00 to Lev15.

As a result, the number of times of the verify operation in the case ofthe normal verify increases to 544 in total, and this is disadvantageousin decreasing the program time at the time of the data write operation.

Voltage Waveform of the Select Word Line

Further, referring to FIG. 10, the voltage waveform of the select wordline in the case of the normal verify is described. The sixth programloop (Prog Loop 6) and seventh program loop (Prog Loop 7), which areindicated by arrows in FIG. 9, are taken as an example.

As shown in FIG. 10, for example, in the sixth program loop (Prog Loop6), a write voltage VPGM is applied to the select word line from a timepoint t1 to a time point t2 (Program).

Then, from a time point t3 to a time point t4 (Verify), the verifyoperation is successively executed for all the threshold voltage levelsLev00 to Lev15.

Subsequently, in the seventh program loop (Prog Loop 7), a write voltageVPGM+dVPGM, which is stepped up from the above-described write voltageVPGM by dVPGM, is applied to the select word line from a time point t5to a time point t6 (Program).

Then, after a time point t7, like the period between the time point t3and time point t4, the verify operation is successively executed for allthe threshold voltage levels Lev00 to Lev15.

In this manner, the voltage, which is applied to the select word line ofthe NAND flash memory, is the repetition of the write voltage (Program)and verify voltage (Verify). In the multilevel NAND flash memory, mostof the program time is occupied by the verify time, and how to reducethe number of times of verify is important from the standpoint ofshortening of the program time (tPROG). In the normal verify, since theverify operation needs to be successively executed for all the thresholdvoltage levels Lev00 to Lev15, the number of times of verify increases,and this is disadvantageous in reducing the program time.

According to the data write operation by the verify skip in theabove-described example, however, a predetermined number of times ofverify can be skipped, and this is advantageous in reducing the programtime (tPROG).

3. Re: Relationship Between the Number of Times of Data Rewrite and theNumber of Times of Verify

Next, referring to FIG. 11 to FIG. 16, a description is given of therelationship between the number of times of data rewrite and the numberof times of verify.

3-1. Relationship Between the Number of Times of Application of WriteVoltage and the Threshold Voltage

The relationship between the number of times of application of writevoltage (Program Loop) and the threshold voltage (Cell Vth) is as shownin FIG. 11. A broken line in FIG. 11 indicates initial characteristics(“Initial”) (e.g. at the time of shipment), and a solid line indicatescharacteristics (“After W/E”) after the repetition of data rewrite (datawrite/erase).

As shown in FIG. 12, the write voltage in this case has a value that isstepped up with a fixed width (dVPGM). In FIG. 12, the abscissa in FIG.11 is changed from the number of times of application of write voltageto the write voltage. As in FIG. 11, a broken line in FIG. 11 indicatesinitial characteristics (Initial) (e.g. at the time of shipment), and asolid line indicates characteristics (After W/E) after the repetition ofdata rewrite (data write/erase).

As shown in FIG. 11 and FIG. 12, the relationship between the number oftimes of application of write voltage and the threshold voltage isproportional (linear), regardless of the initial characteristics(Initial) and the characteristics after data write/erase (After W/E).The reason for this is that the amount of electrons, which are injectedin the floating gate electrode by a single data write operation, isuniformized by properly setting the step-up width of the write voltage.Further, it is understood that after the data write/erase (After W/E),the threshold voltage tends to easily rise, with reference to the numberof times of application of the same write voltage.

3-2. Re: the Number of Times of Verify

Taking the relation of the above section 3-1 into account, the number oftimes of verify, which is finally determined, is explained.

Re: Verify State

In order to explain the verify state, the verify state is divided intothree phases (Phase V to Phase O) as shown in FIG. 13.

The phase V (Phase V) shown in FIG. 13 is a state prior to theoccurrence of a bit at which data write is completed. Specifically, inthis phase V (Phase V), in the bit scan circuit 22, for example, thescanning indicates that the number of bits at which a write operation isbeing executed is N_(B), and the number of bits at which data write isfinished is 0.

Subsequently, the phase X (Phase X) is the state in which data write isbeing executed. Specifically, in the phase X (Phase X), in the bit scancircuit 22, for example, the scanning indicates that the number of bitsat which a write operation is being executed is N_(B)-n, and the numberof bits at which data write is finished is n.

Subsequently, the phase O (Phase 0) is the state in which data write isfinished. Specifically, in this phase (Phase 0), in the bit scan circuit22, for example, the scanning indicates that the number of bits at whicha write operation is being executed is 0, and the number of bits atwhich data write is finished is N_(B).

The Number of Times of Verify in the Case of “Initial”

The number of times of verify in the case of “Initial” is as shown inFIG. 14. FIG. 14 shows an example in which the prescribed number N_(L)is set at N_(L)=8, on the basis of the initial characteristics (Initial)of data write of the memory cell transistor.

As shown in FIG. 14, for example, in the case of “Initial”, the verifyof the threshold voltage level Lev00 begins to pass (Phase X) from the13th program loop (Prog Loop 13).

The Number of Times of Verify after Data Write/Erase

The number of times of verify after data write/erase (“AfterWrite/Erase”) is, for example, as shown in FIG. 15. FIG. 15 shows anexample in which the prescribed number N_(L) is set at N_(L)=3, on thebasis of the write characteristics of the memory cell transistor afterdata write/erase (After Write/Erase).

As shown in FIG. 15, it is understood that, compared to the case of FIG.14, the timing of appearance of the bit at which data write is finishedshifts in such a direction that the number of program loops is smaller.After the repetition of data write/erase, the write timing becomesearlier due to the variation in cell characteristics, and, for example,the verify of the threshold voltage level Lev00 begins to pass (Phase X)from the ninth program loop (Prog Loop 9).

The Number of Times of Verify that is Finally Determined (Verify Skip)

Taking the variations of cell characteristics shown in FIG. 14 and FIG.15 into account, the finally determined number of times of verify of theverify operation (Verify Skip) is as shown in FIG. 16.

As shown in FIG. 16, as regards the finally determined number of timesof verify, in consideration of the fact that the write timing becomesearlier due to the repetition of data write/erase as shown in FIG. 15,the “Loop Number for starting Verify: Prescribed Number N_(L)” in theabove-described step S1-2 needs to be set at a low value.

As described in the above sections 3-1 and 3-2, the verify skipoperation, compared to the normal verify operation, is advantageous inthat the number of times of verify can be reduced. However, there may bea case in which the number of times of verify increases, since it isnecessary to set the verify start point to the lower loop number side inconsideration of the cell degradation after data write/erase, and toshift the number of times of verify, as a whole, to the lower side.

An improved verify skip (hereinafter referred to as “Modified VerifySkip”), in which the verify skip that has been described in the abovesections 2-1 and 2-2 is improved, is explained in the following sections4-1 and 4-2.

4. Data Write Operation Modified Verify Skip

4-1A. Data Write Operation

Referring to FIG. 17A to FIG. 21, a description is given of the“Modified Verify Skip” in the data write operation of the nonvolatilesemiconductor memory device according to the first embodiment. Thedescription below is given with reference to a flow chart of FIG. 17A.The data write operation, which is to be described below, is animprovement (Modified Verify Skip) of the data write operation in theverify skip that has been described in the above sections 2-1 and 2-2.

(Step S2-1 (Program))

To start with, as shown in FIG. 17A, the control circuit 17 applies awrite pulse, on the basis of write data which is input from the externalhost device via the data input/output terminal 15.

(Step S2-2 (Verify Lowest Level))

Subsequently, the control circuit 17 verifies only the lowest thresholdvoltage level Lev00, and does not execute (“skip”) the verify of theother threshold voltage levels Lev00 to Lev15.

(Step S2-3 (Count Verify Pass Bit))

Then, the bit scan circuit 22 counts the bit number of the verify pass(Phase X) of the lowest threshold voltage level Lev00.

(Step S2-4 (Pass Bit>N_(B)?))

Subsequently, the control circuit 17 determines whether the verify passbit number (Pass Bit) of the lowest threshold voltage level Lev00 is theprescribed bit number (N_(B)) or more.

In this step, if the verify pass bit number (Pass Bit) of the lowestthreshold voltage level Lev00 is the prescribed bit number (N_(B)) ormore (Yes), the process advances to step S2-6.

In the meantime, the value of the prescribed bit number N_(B) may be afixed value that is determined at the time of a test step beforeshipment, or may be a variable value which can be re-set after shipment.

(Step S2-5 (VPGM++, PL++))

If the verify pass bit number (Pass Bit) of the threshold voltage levelLev00 is not the prescribed bit number (N_(B)) or more (No), the controlcircuit 17 steps up the write voltage VPGM in the same manner asdescribed above, and increments the number of write loops by one,following which the above-described steps S2-1 to S2-3 are executed onceagain.

(Step S2-6 (Program))

If the verify pass bit number (Pass Bit) of the threshold voltage levelLev00 is the prescribed bit number (N_(B)) or more (Yes), the controlcircuit 17 applies write pulses on the basis of the write data that isinput from the external host device via the data input/output terminal15.

(Step S2-7 (Using Modified Verify Skip))

Subsequently, the control circuit 17 controls the verify operation, anddetermines whether the verify operation is completed or not. Theimproved verify operation (Modified Verify Skip), which is executed atthis time, will be described later in detail.

If the verify operation can be completed (Pass), the data writeoperation is finished.

(Step S2-8 (VPGM++, PL++))

If the verify operation cannot be completed (Fail), the control circuit17 steps up the write voltage VPGM in the same manner as describedabove, and increments the number of write loops by one, following whichthe above-described steps S2-6 to S2-7 are executed once again.

4-1B. Data Write Operation (Modification 2)

Referring to FIG. 17B, a description is given of the “Modified VerifySkip” in the data write operation of the nonvolatile semiconductormemory device according to the modification 2. The description below isgiven with reference to a flow chart of FIG. 17B.

(Step S2-1 (Verify Lowest Level))

To start with, as shown in FIG. 17B, the control circuit 17 verifiesonly the lowest threshold voltage level Lev00, and does not execute(“skip”) the verify of the other threshold voltage levels Lev00 toLev15.

(Step S2-2 (Count Verify Pass Bit))

Then, the bit scan circuit 22 counts the bit number of the verify pass(Phase X) of the lowest threshold voltage level Lev00.

(Step S2-3 (Pass Bit>N_(B)?))

Subsequently, the control circuit 17 determines whether the verify passbit number (Pass Bit) of the lowest threshold voltage level Lev00 is theprescribed bit number (N_(B)) or more.

In this step, if the verify pass bit number (Pass Bit) of the lowestthreshold voltage level Lev00 is the prescribed bit number (N_(B)) ormore (Yes), the process advances to step S2-6.

In the meantime, the value of the prescribed bit number N_(B) may be afixed value that is determined at the time of a test step beforeshipment, or may be a variable value which can be re-set after shipment.

(Step S2-4 (Program))

If the verify pass bit number (Pass Bit) of the threshold voltage levelLev00 is not the prescribed bit number (N_(B)) or more (No), the controlcircuit 17 applies a write pulse, on the basis of write data which isinput from the external host device via the data input/output terminal15.

(Step S2-5 (VPGM++, PL++))

Subsequently, the control circuit 17 steps up the write voltage VPGM inthe same manner as described above, and increments the number of writeloops by one, following which the above-described steps S2-1 to S2-3 areexecuted once again.

(Step S2-6 (VPGM++, PL++))

If the verify pass bit number (Pass Bit) of the threshold voltage levelLev00 is the prescribed bit number (N_(B)) or more (Yes), the controlcircuit 17 steps up the write voltage VPGM in the same manner asdescribed above, and increments the number of write loops by one.

(Step S2-7 (Program))

Subsequently, the control circuit 17 applies write pulses on the basisof the write data that is input from the external host device via thedata input/output terminal 15.

(Step S2-8 (Using Modified Verify Skip))

Subsequently, the control circuit 17 controls the verify operation, anddetermines whether the verify operation is completed or not. Theimproved verify operation (Modified Verify Skip), which is executed atthis time, will be described later in detail.

If the verify operation cannot be completed (Fail), the above-describedsteps S2-6 to S2-7 are executed once again by the control circuit 17.

If the verify operation can be completed (Pass), the data writeoperation is finished (End).

4-2. Improved Verify Skip (Modified Verify Skip)

Next, referring to FIG. 18 to FIG. 21, a detailed description is givenof the above-described improved verify skip (Modified Verify Skip).

The Number of Times of Verify in the Improved Verify Operation Accordingto the Present Embodiment (Modified Verify Skip)

Taking into account the variation of cell characteristics shown in FIG.19 and FIG. 20 which are described later, the number of times of verifyof the improved verify operation (Modified Verify Skip) according to thepresent embodiment is as shown in FIG. 18.

As shown in FIG. 18, for example, as regards the number of times ofverify in the modified verify operation according to the presentembodiment, the write timing of the data write/erase becomes earlier,and the verify at the lowest threshold voltage level Lev00 begins topass (Phase X) from the 11th program loop (Prog Loop 11).

As regards the threshold voltage level at which data write is finished,no verify operation is executed thereafter. Specifically, if the verifystate transitions to the state O (phase 0) in FIG. 18, no verifyoperation is executed. For example, since the data write at the lowestthreshold voltage level Lev00 is completed in the 17th program loop(Prog Loop 17), no verify operation is executed from the subsequent 18thprogram loop (Prog Loop 18).

As a result, the number of times of verify in the modified verify skipin this embodiment can be decreased to 107 in total. Thus, compared tothe case of the verify skip, the number of times of verify can furtherbe reduced, and the program time (tPROG) can advantageously be reduced.

The Number of Times of Verify in the Case of “Initial”

The number of times of verify in the case of “Initial” is as shown inFIG. 19.

As shown in FIG. 19, for example, in the case of “Initial”, the verifyof the threshold voltage level Lev00 begins to pass (Phase X) from the13th program loop (Prog Loop 13).

The number of times of verify in the modified verify skip in thisembodiment in the case of “Initial” is 109 in total.

The Number of Times of Verify after Data Write/Erase

The number of times of verify after data write/erase (After Write/Erase)is as shown in FIG. 20.

As shown in FIG. 20, it is understood that, compared to the case of FIG.19, the write becomes earlier to the lower threshold voltage level side.For example, in the case of the characteristics after the datawrite/erase, the write timing becomes earlier due to the variation ofcell characteristics, which is caused by the data write/erase, and theverify at the threshold voltage level Lev00 begins to pass (Phase X)from the ninth program loop (Prog Loop 9).

The number of times of verify in the case of the modified verify skipafter the data write/erase in this embodiment is 105 in total.

As described above, according to the modified verify skip, even in thecase where the write timing becomes earlier due to the variation of cellcharacteristics, which is caused by the data write/erase, it is possibleto suppress the increase in the final number of times of verify.

Voltage Waveform of the Select Word Line

Further, referring to FIG. 21, the voltage waveform of the select wordline in the case of the modified verify skip is described. The 17thprogram loop (Prog Loop 17) and 19th program loop (Prog Loop 19), whichare indicated by arrows in FIG. 18, are taken as an example.

As shown in FIG. 21, for example, in the 17th program loop (Prog Loop17), a write voltage VPGM is applied to the select word line from a timepoint t1 to a time point t2 (Program).

Then, from a time point t3 to a time point t4 (Verify), the verifyoperation is successively executed for all the six threshold voltagelevels Lev01 to Lev06.

Subsequently, in the 18th program loop (Prog Loop 18), a write voltageVPGM+dVPGM, which is stepped up from the above-described write voltageVPGM by dVPGM, is applied to the select word line from a time point t5to a time point t6 (Program).

Then, from a time point t7 to a time point t8 (Verify), the verifyoperation is successively executed for all the six threshold voltagelevels Lev02 to Lev07.

Subsequently, in the 19th program loop (Prog Loop 19), a write voltageVPGM+2×dVPGM, which is stepped up from the above-described write voltageVPGM by 2×dVPGM, is applied to the select word line from a time point t9to a time point t10 (Program).

Then, from a time point t11 to a time point t12 (Verify), the verifyoperation is successively executed for all the six threshold voltagelevels Lev03 to Lev08.

In this manner, since the verify time, which occupies most of theprogram time, can be decreased, it is obvious that the program time(tPROG) can be made shorter than in the case of the normal verify shownin FIG. 10.

In addition, the program time (tPROG) can more advantageously be reducedsince a predetermined number of times of verify can be omitted (“Skip”),even in the case where the write timing becomes earlier than in theabove-described verify skip due to the variation of the cellcharacteristics by the data write/erase.

5. Advantageous Effects of the First Embodiment

At least the following advantageous effects (1) and (2) can be obtainedby the semiconductor memory device according to the first embodiment andthe data write method thereof.

(1) The program time (tPROG) can advantageously be reduced.

(1)-1. Re: Verify Skip

As has been described above, in the semiconductor memory deviceaccording to the first embodiment, at the time of the step S1-2, thecontrol circuit 17 determines whether the number of times of applicationof write pulses, PL, is the prescribed number N_(L) or not, inaccordance with the relationship between the number of program loops andthe threshold voltage level for verify, which is shown in FIG. 16. Then,the control circuit 17 executes control to perform the verify operation(Verify Skip) for skipping predetermined verify operations.

For example, as shown in FIG. 8, compared to the normal verify (NormalVerify) in which a verify operation is executed for all thresholdvoltage levels, Verify VC1 and Verify VC2, which are indicated by brokenlines in FIG. 8, can be omitted (“Skip”).

For example, in the case of the present embodiment, under thesupposition that the write at the threshold voltage level Lev00 will befinished from the fourth program loop (Prog Loop 4), a verify operationis executed for only the threshold voltage level Lev00 at the fourthprogram loop. On the other hand, at this time, the verify operation forthe other threshold voltage levels Lev01 to Lev15 is not executed and isskipped.

For example, in the same manner as described above, at the time of thefifth program loop (Prog Loop 5), the verify operation is executed foronly the threshold voltage levels Lev00 and Lev01. On the other hand, atthis time, the verify operation for the other threshold voltage levelsLev02 to Lev15 is not executed and is skipped.

For example, at the time of the eighth program loop (Prog Loop 8), theverify operation is executed for the threshold voltage levels Lev00 toLev04. On the other hand, the verify operation for the other thresholdvoltage levels Lev05 to Lev15 is not executed and can be skipped.

Subsequently, in the same manner as described above, as the number ofprogram loops increases, the threshold level for the verify operation isincreased.

As a result, as described above, the number of times of verify in theverify skip in this embodiment can be set at, e.g. 309 in total.

On the other hand, in the normal verify (Normal Verify), the verifyoperation needs to be executed for all threshold voltage levels Lev00 toLev15. Thus, the number of times of verify increases to 544 in total,and this is disadvantageous in shortening the program time.

As shown in FIG. 10, the voltage, which is applied to the select wordline at the time of data write, is the repetition of the write voltage(Program) and verify voltage (Verify). In the multilevel NAND flashmemory, most of the program time is occupied by the verify time, and howto reduce the number of times of verify is important from the standpointof shortening of the program time. In the normal verify, since theverify operation needs to be successively executed for all the thresholdvoltage levels Lev00 to Lev15, the number of times of verify increases,and this is disadvantageous in reducing the program time.

However, according to the data write operation by the above-describedverify skip, since a predetermined number of times of verify can beomitted (“Skip”), the program time (tPROG) can advantageously bereduced.

(1)-2. Re: Modified Verify Skip

The verify skip operation in the above section (1)-1, compared to thenormal verify operation, is advantageous in that the number of times ofverify can be reduced. However, in some cases, the number of times ofverify may increase by such a degree that the verify start point has tobe set to the lower loop number side in consideration of the celldegradation after data write/erase.

In the modified verify skip which has been improved in consideration ofthe above point, the following steps are performed.

(I) In step S2-2, according to the relationship shown in FIG. 18, thecontrol circuit 17 executes control to start the verify from the firstprogram loop (Prog Loop 1) with respect to only the lowest thresholdvoltage level (Lev00 in the present example), and skips the verify ofthe other threshold voltage levels (Lev01 to Lev15 in this example).

(II) Then, in step S2-3, the bit scan circuit 22 counts the bit numberof the verify pass (Phase X) of the lowest threshold voltage levelLev00. In the subsequent step S2-4, on the basis of the count number ofthe bit scan circuit 22, the control circuit 17 determines whether theverify pass bit number (Pass Bit) of the lowest threshold voltage levelLev00 is the prescribed bit number (N_(B)) or more.

(III) In step S2-6, if the verify pass bit number (Pass Bit) of thethreshold voltage level Lev00 is the prescribed bit number (N_(B)) ormore (Yes), the control circuit 17 applies write pulses on the basis ofthe write data that is input from the external host device via the datainput/output terminal 15. In step S2-7, the verify of the next thresholdvoltage level (Lev01 in this example) is started from the next programloop.

(IV) Subsequently, in step S2-7, the end of verify at each thresholdvoltage level is detected. If the verify is finished, the subsequentverify operation is skipped (“Skip”).

In the verify operation according to the modified verify skip, as shownin FIG. 18, regardless of “Initial” (FIG. 19) and “After W/E” (FIG. 20),the optimal number of times of verify can automatically be selected, thewrite speed can be increased and stabilized, and therefore thereliability can be enhanced.

For example, in the case of the present embodiment, as shown in FIG. 18,the verify number in the modified verify skip can be set at 109 intotal. Thus, compared to the above-described verify skip, the number oftimes of verify can further be reduced, and the program time canadvantageously be shortened.

In the modified verify skip according to this embodiment, as shown inFIG. 18, the number of times of verify and the verify start point can beadjusted in the verify operation in accordance with the write speed ofthe memory transistor. By this modified verify skip, the number of timesof verify of the NAND flash memory can greatly be reduced, and theprogram speed (MB/sec) can be improved.

Since the number of times of verify and the verify start point can bedetermined as shown in FIG. 18, if it can be confirmed by the bit scancircuit 22 that the pass bit number of the lowest threshold voltagelevel (Lev00 in this example) is the prescribed bit number N_(B) or more(steps S2-2 to S2-4), then it should suffice to execute verify in aninverted N shape by making use of the characteristics of the thresholdvoltage, which are shown in FIG. 11 and FIG. 12. The inverted N shaperefers to a voltage waveform in such a series of operations that afterprogram pulses are applied, verify pulses are applied stepwise inaccordance with threshold voltage levels, and then the stepped-upprogram pulses are applied, as shown in FIG. 9 and FIG. 10. For example,as shown in FIG. 10, in the sixth program loop (Prog Loop 6), the writevoltage pulse VPGM is applied to the select word line from time point t1to time point t2 (Program). Subsequently, the verify pulse issuccessively applied with respect to all the threshold voltage levelsLev00 to Lev15 during the period (Verify) between time point t3 and timepoint t4. Then, in the seventh program loop (Prog Loop 7), thestepped-up write voltage pulse VPGM+dVPGM is applied during the period(Program) between time point t5 and time point t6.

(2) The multilevel implementation can advantageously be achieved.

With the development of the multilevel implementation of the memory, thenumber of threshold levels, which require the verify operation,increases, for example, as shown in FIG. 10. As shown in FIG. 10, in themultilevel NAND flash memory, most of the program time is occupied bythe verify time.

However, as has been described in the above sections (1)-1 and (2)-2,according to the first embodiment, the number of times of verify can bereduced while the reliability can be maintained.

Therefore, the multilevel implementation of the memory canadvantageously be achieved.

Second Embodiment An Example in which Data Write is Executed from aCentral Threshold Voltage Level Side

Next, referring to FIG. 22 to FIG. 25, a description is given of asemiconductor memory device according to a second embodiment of theinvention and a data write method thereof. This embodiment relates to anexample in which the control circuit 17 executes control to perform datawrite from a threshold voltage level (the central 8th value Lev08 inthis example) side, which is higher than the lowest threshold voltagelevel (Lev00) of the multilevel data (16 values Lev00 to Lev15 in thisexample). In this description, a detailed description of the partscommon to those in the first embodiment is omitted.

6. Data Write Operation

Referring to FIG. 22 to FIG. 26, a description is given of the datawrite operation of the semiconductor memory device according to thesecond embodiment.

The second embodiment differs from the first embodiment in that in theabove-described step S2-2, the data write is started at the thresholdvoltage level Lev08 that is the central value, as indicated at the 0thprogram loop (Prog Loop0) in FIG. 22.

Then, as shown in FIG. 23, the verify is executed only at the thresholdvoltage level Lev08 from the first program loop (Prog Loop 1) to the20th program loop (Prog Loop 20). On the other hand, between the firstprogram loop (Prog Loop 1) and the 20th program loop (Prog Loop 20), theverify operation is not executed (“Skip”) for the other thresholdvoltage levels Lev00 to Lev07, and Lev09 to Lev15.

Subsequently, as shown in FIG. 24, in the 22nd program loop (Prog Loop22), the verify is executed for the threshold voltage levels Lev08 andLev09. The verify operation is not executed (“Skip”) for the otherthreshold voltage levels Lev00 to Lev07, and Lev10 to Lev15, as in theabove-described case.

Subsequently, in the similar manner as described above, the verifyoperation for the threshold voltage level Lev15 is first executed untilthe 33rd program loop (Prog Loop 33).

Then, as shown in FIG. 25, the verify is executed only at the lowestthreshold voltage level Lev00 from the 34th program loop (Prog Loop 34).The verify operation is not executed (“Skip”) for the other thresholdvoltage levels Lev01 to Lev15. At this time, for example, the verify isexecuted by restoring the write voltage VPGM to about 10 V, whichcorresponds to the lowest threshold voltage level Lev00. At this time,in the above-described step S2-7, even in the case where the verify forthe threshold voltage level Lev15 has passed, the data write is notfinished, and the operation of the step S2-2 is started. In thisrespect, the second embodiment differs from the first embodiment.

Subsequently, the same data write operation as in the first embodimentis executed from the 35th program loop (Prog Loop 35).

As a result, as shown in FIG. 26, at the 67th program loop (Prog Loop67), 16-value multilevel data having a desired threshold voltagedistribution can be written.

7. Advantageous Effects

As has been described above, at least the same advantageous effects (1)and (2) as described above can be obtained by the semiconductor memorydevice according to the second embodiment and the data write methodthereof.

Furthermore, the second embodiment differs from the first embodiment inthat the control circuit 17 executes control to perform data write fromthe central threshold voltage level (the 8th value Lev08 in thisexample) side of the multilevel data (16 values Lev00 to Lev15 in thisexample).

Thus, more advantageously, the data retention characteristics of the lowthreshold voltage levels (Lev00 to Lev07 in this example) at which datawrite is executed later can be improved.

This control is applicable as needed.

In the second embodiment, the case in which data write is executed fromthe threshold voltage level Lev08, which corresponds to the centralvalue, is taken as an example, but the invention is not limited to thisexample. Specifically, it should suffice if the data write is startedfrom a threshold voltage level side, which is higher than the lowestthreshold voltage level (Lev00). For example, the scope of the secondembodiment covers the case in which the data write is started from thethreshold voltage level Lev10 or the highest threshold voltage level(Lev15 in this example).

In the above description, 16 values have been described as an example ofmultilevel values, but the invention is not limited to this example. Theinvention is similarly applicable to not only the 16-value NAND flashmemory, but also 4-value or 8-value NAND flash memories, and the sameadvantageous effects can be obtained. Besides, the invention isapplicable to not only the multilevel NAND flash memory, but alsosingle-level NAND flash memory (SLC: single-level cell) which can store1-bit data in one memory cell transistor.

In the embodiments, data write of random data has mainly been described.However, the invention is applicable to the case of data write ofspecific data (e.g. write of only “LevF” data).

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A semiconductor memory device comprising: a memory cell arrayincluding a plurality of memory cells, the plurality of memory cellsbeing arranged in a matrix; and a control circuit configured to controlperforming a verify operation with respect to a lowest threshold voltagelevel of a memory cell in a data write operation, and skipping theverify operation with respect to the other threshold voltage levels. 2.The device according to claim 1, wherein the control circuit determines,in the data write operation, whether a verify pass bit number of thelowest threshold voltage level, which is counted by a bit scan circuit,is a prescribed bit number or more, and the control circuit furtherexecutes control, if the verify pass bit number is the prescribed bitnumber or more, to perform the verify operation with respect to only thelowest threshold voltage level and a threshold voltage level that ishigher than the lowest threshold voltage level, and to skip the verifyoperation with respect to the other threshold voltage levels.
 3. Thedevice according to claim 1, wherein the control circuit furthercontrols to skip a subsequent verify operation of a threshold voltagelevel at which data write is finished.
 4. The device according to claim1, further comprising a memory which is electrically connected to abuffer and stores write/read data which is input from outside.
 5. Thedevice according to claim 1, further comprising a column decoder whichis electrically connected to a bit line control circuit and selects datain a column direction.
 6. The device according to claim 1, wherein thememory cell has a multi-layer structure including a tunnel insulationfilm, a charge accumulation layer, an inter-gate insulation film and acontrol electrode layer, which are successively provided on asemiconductor substrate.
 7. A semiconductor memory device comprising: amemory cell array including a plurality of memory cells, the pluralityof memory cells being arranged in a matrix and capable of storingmulti-bit data; and a control circuit configured to control performing averify operation with respect to a first threshold voltage level whichis higher than a lowest threshold voltage level of a memory cell in atime of a data write operation, and skipping the verify operation withrespect to another threshold voltage level which is higher than thefirst threshold voltage level.
 8. The device according to claim 7,wherein the control circuit determines, in the data write operation,whether a verify pass bit number of the first threshold voltage level,which is counted by a bit scan circuit, is a prescribed bit number ormore, and the control circuit further executes control, if the verifypass bit number is the prescribed bit number or more, to perform theverify operation with respect to the first threshold voltage level and asecond threshold voltage level that is higher than the first thresholdvoltage level, and to skip the verify operation with respect to theother threshold voltage levels.
 9. The device according to claim 8,wherein in the time of the data write operation, the control circuitcompletes the verify operation of a highest threshold voltage levelafter finishing the verify operation of the second threshold voltagelevel, and then performs the verify operation with respect to only thelowest threshold voltage level and skips the verify operation withrespect to the other threshold voltage levels, the control circuitdetermines whether a verify pass bit number of the lowest thresholdvoltage level, which is counted by the bit scan circuit, is a prescribedbit number or more, and the control circuit further executes control, ifthe verify pass bit number of the lowest threshold voltage level is theprescribed bit number or more, to perform the verify operation withrespect to only the lowest threshold voltage level and a thresholdvoltage level that is higher than the lowest threshold voltage level,and to skip the verify operation with respect to the other thresholdvoltage levels.
 10. The device according to claim 7, wherein the controlcircuit further controls to skip a subsequent verify operation of athreshold voltage level at which data write is finished.
 11. The deviceaccording to claim 7, further comprising a memory which is electricallyconnected to buffer and stores write/read data which is input fromoutside.
 12. The device according to claim 7, further comprising acolumn decoder which is electrically connected to a bit line controlcircuit and selects data in a column direction.
 13. The device accordingto claim 7, wherein the memory cell has a multi-layer structureincluding a tunnel insulation film, a charge accumulation layer, aninter-gate insulation film and a control electrode layer, which aresuccessively provided on a semiconductor substrate.
 14. The deviceaccording to claim 1, further comprising: a memory cell unit includingthe plurality of memory cells connected in series; word lineselectrically connected to gate electrodes of the plurality of memorycells, respectively; bit lines electrically connected to one end of thememory cell unit; a word line driving circuit which applies a voltage tothe word lines; and a bit line control circuit; wherein the controlcircuit controls the word line driving circuit and the bit line controlcircuit.
 15. The device according to claim 7, further comprising: amemory cell unit including the plurality of memory cells connected inseries; word lines electrically connected to gate electrodes of theplurality of memory cells respectively; bit lines electrically connectedto one end of the memory cell unit; a word line driving circuit whichapplies a voltage to the word lines; and a bit line control circuit;wherein the control circuit controls the word line driving circuit andthe bit line control circuit.
 16. The device according to claim 1,wherein the control circuit is further configured to control performinga verify operation with respect to a first threshold voltage level whichis higher than a lowest threshold voltage level of the memory cell inthe data write operation, and skipping the verify operation with respectto the other threshold voltage levels except for the lowest thresholdvoltage level and the first threshold voltage level.
 17. The deviceaccording to claim 7, wherein the control circuit is further configuredto control performing a verify operation with respect to a secondthreshold voltage level which is higher than a lowest threshold voltagelevel of the memory cell and differs from the first threshold voltagelevel in a data write operation, and skipping the verify operation withrespect to the other threshold voltage levels except for the firstthreshold voltage level and the second threshold voltage level.